Laser diodes are used as transmitter elements in fiber optic communications systems such as e.g. PONs (Passive Optical Networks). Laser light power is required to remain on constant levels for logical zero and logical one during operation. However, the characteristics of laser diodes show strong temperature dependence. Further, they are subject to variations during aging. Therefore, the drive current of a laser diode has to be controlled to achieve constant levels of light power during operation.
Since the temperature of the laser diode and the drive current required to attain a certain level of light power do not have a linear relationship, it is difficult to design a control circuit that implements accurate control. Typically, a monitor diode is used to monitor the output power of the laser diode and the drive current of the laser diode is controlled in feedback loop operation to provide for constant power laser output.
A drive current regulation based on feedback control needs a convergence time to reach steady state drive current conditions. The convergence time may cause information loss in the optical network. Since control data such as e.g. header data defining the destination of a message is frequently transmitted at the beginning of laser operation, a short convergence time of the feedback control laser driver may be essential for a high data throughput in the communications system.
Further, laser diode aging is a threat to the stability of an optical network. Since drifts of the laser diode drive current due to aging are non-deterministic, sudden variations of drive current may occur. Further, variations of the drive current due to aging and variations of the drive current due to temperature effects can often not be separated during drive current feedback control.